Resonance absorption apparatus and method for measuring magnitude and direction of a magnetic field



Nov. 24, 1964 P. JUNG ETAL 3,158,802

RESONANCE ABSORPTION APPARATUS AND METHOD FOR MEASURING MAGNITUDEZ ANDDIRECTIQN OF A MAGNETIC FIELD Filed Nov. 12, 1959 5 Sheets-Sheet 1 abs IL flujgt'b INVENTORS 0 U PAUL JUNG JEAN VAN CAKENBERGHE Nov. 24, 1964 P.JUNG ETAL 3,158,802

RESONANCE ABSORPTION APPARATUS AND METHOD FOR MEASURING MAGNITUDE ANDDIRECTION OF A MAGNETIC FIELD Filed Nov. 12, 1959 5 Sheets-Sheet 2 MamMax.2 Min.

M i\/ a t O l l l t 6 ifi m PAUL JUNG JEAN VAN CAKENBER'GHE Nov. 24,1964 P. JUNG ETAL 3,158,802

RESONANCE ABSORPTION APPARATUS AND METHOD FOR MEASURING MAGNITUDE ANDDIRECTION OF A MAGNETIC FIELD Filed Nov. 12, 1959 5 Sheets-Sheet 5 4 l01 Y Y W Paramagneflc X Resonance x V Umf I X-Y Separator anq Feedback /41 Umr A.F. Generator and Reference- Phase Unif BY M ATTOR EY N v- 1964P. JUNG ETAL 3,158,802 N APPARATUS AND METHOD FOR MEASURING RESONANCEABSORPTIO lglgGNITUDE AND DIRECTION OF A MAGNETIC FIELD Filed Nov. 12

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E H S G V, R R E m E I N U m m UM 55313 3334 23 M KA G c NN NW" LN UA AEPJ Nov. 24, 1964 P. JUNG ETAL 3,158,802

RESONANCE ABSORPTION APPARATUS AND METHOD FOR MEASURING MAGNITUDE ANDDIRECTION OF A MAGNETIC FIELD Filed Nov. 12, 1959 5 Sheets-Sheet 5 a I IJ: 2'0 i I Paramagnefic I Resonance H 12 1 Unit i I X-Y Separator anqFeedback 14 l Unn M I \i i I Aifienerafor an e erence- Phase Unit 2Channel JNVENTORS PAUL JUNG JEAN VAN CAKENBERGHE United States PatentOfi ice 3,153,8fi2 Patented Nov. 24, 1964 The present invention relatesin general to the determination of magnetic fields and moreparticularly, to the determination of the magnitude and direction ofweak magnetic fields.

This is a continuation-impart of our copending United States patentapplication, Serial No. 713,626, filed February 6, 1958, and entitledMagnetometer. In that earlier application, novel apparatus for measuringone component of a magnetic field is disclosed and claimed. Theoperation of the previously disclosed apparatus is based on thesymmetrical characteristics of a periodically reversed magnetic field.The symmetry of this field is destroyed by the external magnetic fieldto be measured, and the resulting asymmetry is detected by analyzing theresonance absorption of a paramagnetic substance.

The apparatus of our earlier filed application is admirably suited fordetermining the intensity of a magnetic field in a given direction (forexample, the Vertical component of the earths magnetic field). However,the complete determination of the magnitude and direction of an externalmagnetic field it, such as the earths magnetic field, requires ingeneral the measurement of the three perpendicular components I h and hAs disclosed in our earlier filed application, the three components canbe measured by using three independent alternating sweep fieldsperpendicular to each other and simultaneously recording theirrespective output signals. The apparatus of the present invention,however, not only measures the intensity of the external magnetic field,but also greatly improves upon the direction determination ability ofour earlier apparatus.

it is, therefore, the main object of the present invention to provide amethod and apparatus for simultaneously measuring the intensity anddirection of a magnetic field.

Another object of the invention is to provide a method and appartus forrapid and simultaneous determination of the amplitude and direction of aWeak magnetic field.

Other aims and advantages of the present invention will be apparent fromthe following description and appended claims.

In the drawings: 1

FIGURE 1 is a diagrammatic representation of the magnitude and directionof a high frequency magnetic field H and a constant or slowly varyingfield H;

FIGURE 2 is a graph of the paramagnetic absorption H and anexternalmagnetic field h at an angle'as with time ac-axis; V

FlGURE 6a is a graph of the magnitude of the resultant field H, as afunction of and FIGURE 623 is a graph or" the absorption oftheparamagnetic substance due to the resultant field;

. filed application. a iunctionwhich is transformed into itself when itsvaria FIGURE 7 is a diagrammatic representation of the resultant fieldH, in a three-dimensional system;

FIGURE 8 shows graphs of the sweep field H the resultant field H and theabsorption in the paramagnetic substance, each respectively as afunction of time;

FIGURE 9 is a diagrammatic representation of the field H", resultingfrom the combination of field H, and the h component of the externalmagnetic field h;

FIGURES 10a, 10b, andlOc show the variation with time of the H, sweepfield, the resultant field H}, and the absorption in the paramagneticsubstance, respectively;

FIGURE 11 is a diagrammatic representation of a twodimensionalembodiment of the present invention;

FIGURE lla is the block diagramof FIGURE 11 rearranged and modified tocorrespond to the circuit diagram of FlGURE 12;

FIGURE 12 is a complete circuit diagram of the ap paratus of FIGURE 11;and

FIGURE 13 is a diagrammatic representation of a three-dimensionalembodiment of the present invention.

it has now been found that instead of using three sweep fields ofarbitrary magnitudes and frequencies, the complete determination of thedirection and magnitude of an external magnetic field can be more easilyachieved by using a rotating field of constant magnitude rotating with aconstant angular velocity w in one plane (x, y) and an alternating fieldof angular velocity w sweeping in the direction transverse to the plane.The combination of these two fields produces the same result as threesweep fields perpendicular to each other, since a rotating field may beproduced by the combination of two alternating fields of the samefrequency and magnitude, but shifted in phase by 7r/2 radians.

he present invention also differs from the apparatus of our earlier fileapplication in that the magnitude of the sweep fields is never allowedto be zero. In our former apparatus the sweep fields did pass through azero value of magnitude. By maintaining the sweep field magnitudedifferent from zero, the technical performance of our earlier describedapparatus is greatly improved in that the detection of the output signaland the electrical circuit are simplified.

It is of importance in the present invention that the magnitude of thesweep field remain smaller than that of the resonance field in orderthat the paramagnetic absorption is an increasing function of the field.

in accordance with tie present invention, there is provided a device formeasuring the magnitude and direction of an external magnetic fieldcomprising a paramagnetic substance, generator means for irn-posiru amagnetic field on the paramagnetic substance, and means for measuringthe asymmetrical variation of the resonance absorption'of theparamagnetic substance as caused by the external mag netic field.

As in the apparatus of our earlier filed application, Serial No.713,626, the determination of the magnitude and dir ction of an unknownmagnetic field, and in particular Weak magne ic fields such as that ofthe earth, depends upon the symmetrical proper ies of the sweep fields.in the absence of an external magnetic field, the paramagneticabsorption is a symmetrical function of the sweep field, as can be seenfrom the absorption patterns in the drawings. However, in the presenceof an unknown external magnetic field, the paramagnetic absorption is nolonger a symmetrical function, but is asymmetrical. The asymmetricalpattern created by the presence of an external field can be seen inFJGURE 100. This asymmetrical variation can be detected'by analyzing theresonance absorption, and the symmetry can be restored by a correctingsignal, such as that described in our earlier By a symmetrical functionis meant a rotating field H should be quasirconstant.

shown inPIG. 4b.

sasasea able is changed from a positive to a negative value or viceversa. 7 a

' A paramagnetic substance, such as a free radical, which is exposed toa high frequency field, H can be made resonant by imposing on it aconstant or a slowly varying field H, the axis of which is perpendicularto the axis of the high frequency field, as shown in FIG. 1. Theparamagnetic absorption is a function of the magnitude of the constantor slowly varying field and can be represented by a bell-shaped curve asshown in FIG. 2. If the high frequency field Hjis not perpendicular tothe field H, the function A=f(H) should be written A=j(H) cos 6 where isthe angle between H and H, as shown in FIG. 1. The field H is a quasiconstant field, i.e., the field H varies at a rate much smaller than therelaxation time of the paramagnetic substance. A more detaileddiscussion of resonance and field dependency of magnetic absorption canbe found in Nuclear Magnetic Resonance by E. R.

7 Andrew, Cambridge University PresaCambridge (195 The sweep fieldfrequencies used to generate a resultant The magnitudes of the sweepfields as well as that of the resulting field should be less than themagnitude of the resonance field H indicated centrally of the bell curveof FIGURE 2; Under these conditions the paramagnetic absorption is anincreasing function of the magnitude of the resulting field 1-1,. in thezone of interest. The zone of interest 'in the present invention isapparent from FIG. 2. When the magnitude of the resulting field Hincreases below the resonance point value H the absorption increases;when the field H, decreases below the resonance point value H theabsorption accordingly decreases.

Either'the rotation of a properly designed magnetic system, or thecombination of two alternating fields of frequency w and of equalmagnitude, but shifted in phase and space by 'Jr/Z with respect to eachother, will produce a rotating magnetic field H '7 As shown in FIG. 3 arotating field in the .t-y plane may be represented by a vector ofconstant magnitude H5, rotating with a constant angular velocity w inthe :r-y

' plane. If the x-axis is in the direction of the rotating field at thereference time i=0, the position of the rotating field at any time t ismeasured by the angle \//=w.t.

Since the magnitude of the rotating field remains con- 7 stant withrespect to time, as shown in EEG. 4a, the corresponding absorption is,constant with respect to time, as

As shown infFIG. 5, if the 11 and h components of an unknown field h atan angle q; with the x-axis interfere in the xy plane, the new field Hresulting from the combination-of H and h (11X, h 0) is no longerconstant with respect to time. As shown in FIG. 5 the combination of Hand hUzp, h 0) results in a vector H the origin of which is at C and theend of which follows the circle centered at B. It is apparent that thevector H changes: in magnitude as it rotates. The magnitude H reaches a7 maximum when the rotating field isaligned with V 7 X7 y; V (point A inFIG. 5) andreaches'a minimum when the rotating field is'in the oppositedirection (point D in FIG. 5). The value of H can be calculated: V

The magnitude of H changes as H changes from {lilo I is no longer ,asymmetrical function'of the In FIGS. 5a .and 6b the time variations sin,and of the'corresponding absorption are shown. The absorption maximumandminim'umare related to IlJEjSPElC angle s of'the unknown field lz (hh by Thus, in a two-dimensional system, the paramagnetic absorption isconstant-and independent of time in the absence of an unknown magneticfield. But in the presence of an unknown magnetic field (i.e., in thepresence of the x and y components of the field to be measured), the

known external magnetic field, sweeping must be per-- formed in threedirections: along the x, y, and z axes. This can be done by using arotating field H in the x-y plane and an alternating sweep field alongthe z-axis.

The frequency of the rotating field should be different from that of thez-sweep field.

If the z-sweep field and the rotat ng field I-l are combined, as shownin FIG. 7, the total sweep field can be represented by the'resultingvector H' The'field H changes its magnitude as the z-field sweeps from+H to H or vice versa. The resulting field passes twice from a maximumvalue to a minimum value H, as the z-field completes a full cycle (O,+H,O,'H IH I is thus a symmetrical function of the z-sweep field, and theabsorption, as shown in FIG. 8, is similarly symmetrical. In FIG. 8,when the z-sweep field completes a cycle in time T, the total (orresulting) field H and the absorption complete a cycle in a time T/2.The absorption corresponding to the combination of a rotating field anda z-sweep field is a periodic function of time, the frequency of whichis two times the frequency of the z-sweep field.

Referring to FIG. 9, it is assumed that an unknown external field isintroduced into the above-described system having a constant magnitudeand direction. As sume only one component of the external magnetic fieldit affects the system and that this component is, I

The resulting field along the aligned along the z-axis. z-aXis will beequal to the sum of the z-sweep field and the Z1 component of theexternal field h(O,O,h when the z-sweep field completes one half-cycle,and to the difference 'of both fields'when the z-sweep field corn pletesthe second half-cycle (in the opposite direction). The. magnitude of thefieldVI-i: is a value which differs according to the. half-cycleconsidered. When the sweep field completes the half-cycle (-()+H ,O),the

magnitude of H'fl, passes from a minimum H to a maximum given by. /H +h+H While for the halfcycle (O,H O) the magnitude of H' passes from H toanother maximum equal to \/H (h -ELF. This is shownin FIGS. 10a, 18b,and 100. The interference of one component ofthe external magnetic fieldintroduces an asymmetrical variation in'the magnitude of the total sweepfield and, the corresponding absorp-v tion is no longer a symmetricalfunction of the sweep field. I I

When each of-the three components .h h hav of the external magneticfield h isidifierent from zero,

the total efiect is determined by'the superimposition of two variations;one of frequency relative to the -an- 7 gular velocityzwt (due,to the zsweepcfield) and a second of fre'quency relative to the angular velocityvg..'(due to.

y the rotating field) "-The totalefiect is determined by the combinationof an w variation resulting from the,

combined etfe'ct of therotating field-with theih anld h, 1'

componeuts lof the external magnetic fieldfandan w variatioresultingfrom the combined efiect of the zsweep field with the h component of theexternal magnetic field.

An electric signal is produced by this absorption variation whichincludes two fundamental frequency components respectively related tothe angular velocities, w and w. The frequency component is related tothe direction of the h 11,, components of the external magnetic field byits phase angle (angle as defined above). The positive or negativedirection-of the 71 component of the external magnetic field is given bythe phase of the fundamental frequency w component with respect to thez-sweep field. The magnitude of the external magnetic field is given bythe magnitude of the two fundamental components w and w.

A slight error is introduced by assuming that the magnitude of the twofundamental frequency components is proportional to the magnitude of theexternal magnetic field, but this error can be eliminated by using anullmethod, as described in our earlier-filed application. Not only canthe external magnetic field be compensated, but the compensating fieldcan be aligned with the direction of the external field. I

The present invention comprises two slightly different apparatus fordetermination of the direction and magnitude of an unknown field, i.e.,a two-dimensional apparatus, which is of particular interest as anauto-pilot for ships, and a three-dimensional apparatus, which can beused in satellites, guide missiles and the like.

Each of these two apparatus has basic equipment which is quite similar.Furthermore, they are similar to the apparatus described in ourearlier-filed application, but with some important difierences.

In the present invention, both paramagnetic and ferromagnetic substancescan be used. Diphenyl-picrylhydrazil, sodium in liquid ammonia, ferritesand garnets, such as yttrium-iron garnet, Y Fe (Fe are operable.

Another important difference from our earlier filed application is thata rotating field H can be obtained either by means of a small permanentmagnet rotating with a suitable angular velocity w or by two lov,frequency alternating fields having the same amplitude and frequencies,but shifted in phase and space by 1.-/ 2 with respect to each other toproduce the desired resultant field H, rotating at angular velocity w.The absolute magnitude of the resultant rotating field H should besmaller than the magnitude of the resonance field H as shown in FIGURE2. The z-sweep field is a low-frequency alternating field having asmaller magnitude than that of the resonance field. The angular velocityw of the z-sweep field should preferably be smaller than the angularvelocity frequency w of the rotating field. It is also preferable that wnot be a multiple or a sub-multiple-of w. The values of w and w shouldbe selected so that their respective harmonics do not substantiallyinterfere with each other.

Referring now to FIGS. 11 and 12, in the twodimensional embodiment ofour invention, the signal from the absorption variation in theparamagnetic substance disposed within the radio frequency coil 10 isdeveloped across a resistance in the radio-frequency oscillator-detectorunit 11 and amplified by an audio-frequency amplifier 12 to a convenientlevel for synchronous detection. The x-component of this signal isdetected by a synchronous detector 13, thereference phase of which isadjusted so as to be sensitive only to the x-cornponent. Synchronousdetector 13 may be followed in the circuitry by a signal booster devicesuch as for example, a D.C. amplifier 13a. A two-phase motor 14 havingjareference phase adjusted so as to be sensitive 'only to the y-component,detects the y-component of the signal from the audio-frequency amplifier12. A signal power amplifier 12a may also be advantageously used toboost the signal power input to the two-phase motor 14.

These reference signals are taken from the low-frequency oscillator 15(AF Advance Generator) through the phase shift networks 16 and 17 andare fed into the synchronous detector 13 and the two-phase motor 14.FIGURES 11a and 12 indicate that x and y reference signal amplificationmay also be incorporated into the circuitry by inclusion of a referencesignal power amplifier 17a. The phase shift of the xand y-components ofthe rotating field is obtained by the phase shift network 18. The powerfor the phase shift networks and for the rotating field system issupplied from the oscillator 15.

The direct current signal from the detector 13 is proportional to thex-component of the fundamental frequency. After amplification, it is fedinto the x-low frequency coil through the current measuring device 19.The signal acting on the two phase motor 14 is proportional to they-component. The motor applies a torque to the alignment system(x-coils, y-coils, and radio frequency coils supported on a platform) bymeans of which the xand y-coils can jointly rotate about the z-axis. Thecoils stop rotating when the x-coil is aligned with the externalmagnetic field h. The y-signal then becomes zero.

Referring now to FIGURE 13, which shows a diagram of thethree-dimensional embodiment of our invention, pieces of apparatus whichare the same as those used in the two-dimensional apparatus are givenidentical reference numerals. In addition, this apparatus includesz-coils which provide the z-sweep field of frequency corresponding toangular velocity w. The w component of the signal is taken from theoscillator-detector 11 or the audio-frequency amplifier 12 at aconvenient level and fed into a z-channel amplifier 20. The signal atthe output of this amplifier is fed into a synchronous detector 21 thereference phase of which is taken from the audiofrequency generator 22through a phase-shift network 23. The generator 22 also supplies powerto the z-coils.

The output current of the synchronous detector 21 is fed back into thez-coils to compensate the z-component of the unknown external magneticfield through a z-current measuring device 24.

To illustrate the operation of the invention, assume the magnitude anddirection of an unknown magnetic field h is to be measured by means ofthe apparatus of the invention and that the magnitude of h is between 1gauss and +1 gauss.

Upon current flowing through the two-dimensional apparatus, it willadjust until a stop-position is reached. The current measuring device 19in FIG. 11 gives the magnitude of the field component in the x-y planewhile the direction of the x-y component is given by the position of thex-coil axis.

The two-dimensional device is intended for measuring only two componentsof the unknown field h, but can be used for measuring the thirdcomponent if needed, provided that the unknown field h is constant inmagnitude and direction for a period of time sufficiently long forproper measurement to be made. The alignment sys tern of the apparatusis, for instance, manually rotated about the y-axis until a maximum isread over the current measuring device. At this position, the x-axis isaligned with the unknown magnetic field and its absolute magnitude is"given by the reading on the current measing device. 7

If the magnetic field to be measured varies in magnitude and/ordirection, measurement should be made by the three-dimensional apparatus(provided that the field variations are slower than the time response ofthe measuring device). Upon switching on the three-dimensionalapparatus, the x-y operation occurs in a manner similar to thetwo-dimensional apparatus. The feed-back current through the z-coilscauses a reading on the z-current meas uring device which isproportional to the z-component of the unknown field h. The totalmagnitude of h is given '7 d by IhP=h +h and the angle 6 between thefield direction andthe x-axis is given by lithe z-cornponent isCompensated by meansof a mechanical feed-back system, e.g., by rotationof the x-y coils about the y-aids, the apparatus will adjust itself to aposition in which the x-axis points in the direction of the unknownfield and its magnitude is given by the reading on the current measuringdevice.

Example I A Workingimodel of the two-dimensional apparatus of thisinvention was constructed using an ordinary magnetic deflection systemfor television tubes as the X and V Ycoils. The field of such a systemis not homogeneous and the results obtained were poor in comparison withthat which could be obtained by proper design.

I One gram of xx diphenyl ,8 picryl-hydrazil was used as theparamagnetic substance. The resonance frequency was 9.3 megacycles andthe frequency of the P1,; and B fields was 400 cycles per second. Thespeed of angular response was 1 radian per second and the minimumdetectable angular changewas l0- radian.

Example II An apparatus similar to that of Example l was constructedexcept that low frequency z-coils were added around the paramagneticsubstance. Performance was similar to that obtained inExample 1 exceptthat'the x- 7 axis aligned itself with the direction of the field.

The apparatus of, this invention measures both magnitude and directionof an unperturbated external magnetic field, that is, theinventivevdevice does not contam fied in order to compensate the efiectof this momentum' upon the external field tobemeasured.

The invention can be used for measuring magnetic fields, preferably weakmagnetic fields (practical range from '0 to about 5 gauss). It is also avery convenient instrument for measuring a zero magnetic field.

Many uses for this invention are apparent. It can be usedin'astrophysics, in particular as magnetometer. or magnetic compass inall types of out-of-space vessels, guided'missiles, artificialsatellites and rockets, in geophysics, in particular geophysicalsurveying, mining prospection, as an auto-pilot for planes, rockets,ships and magnetic detectors.

What is claimed is:

*1. Apparatus for measuring'the magnitude andjdirec 'tion of an externalmagnetic fieldcomprisin a paramagnetic substance having a determinablemagnetic resonance absorbtion frequency and magnitudefcharacterispp. 930and 931'principally relied upon;

tie; first generator means for imposing on said paramagnetic substance ahigh frequency magnetic field, second and having a frequency and amagnitude less respectively than the characteristic magnetic resonanceabsorption frcquency and magnitude of said paramagnetic substance, andthird generator means for imposing on said substance a magnetic field ofconstant magnitude less than the magnetic resonance absorption magnitudeof said paramagnetic substance rotating with a constant angular velocityless than the angular velocity corresponding to the magnetic resonanceabsorption frequency of said paramagnetic substance in a planeperpendicular to the'axis of said low frequency field; and means formeasuring asymmetrical variations in the absorption of said paramagneticsub stance- 2. A method for measuring the magnitude and direction of anexternal magnetic field comprising: subiecting a paramagnetic substancehaving a determinable magnetic resonance absorbtion frequency and'magnitude characteristic to a high frequency magnetic field, a lowfrequency magnetic field parallel to said high frequency field andhaving a frequency and magnitude less respectively than thecharacteristic magnetic resonance ab sorbtion frequency and magnitude ofsaid paramagnetic substance, and a magnetic field of constant magnitudeless than the characteristic 'magnetic resonance absorption magnitudeof. said paramagnetic substance rotating with a constant angularvelocity less than the angular velocity corresponding to thecharacteristic magnetic resonance absorbtion frequency of saidparamagnetic substance in a plane perpendicularto the aids of said lowfrequency field; subjecting said paramagnetic substance to said externalmagnetic field; and obtaining a measurement of the asymmetricalvariations in the absorption of said paramagnetic substanceQ ReferencesQited in the file of this patent UNITED STATES PATENTS OTHER REFERENCESIngram et al.: Philosophical Magazine, vol. No. 370, Nov. 1954, pp. 1221to 1223.

Nertz: Chemical Reviews, vol. 55, No. 5 Oct. 1955,

Marius et al.: Academic desSciences, Comptes Rendus, vol. 239, No. 5,Aug. 1954, pp. 414 and 415. I

15, No.5, May 1954, page 378.

Manus et al. Journal dePhysique et Le Radium, vol,

Pound et al.: The Review of Scientific Instruments,

' v01. 21, No. 3, March, 1950, pp. 219 to 225.

1958, pp. 381' to 385.-

1. APPARATUS FOR MEASURING THE MAGNITUDE AND DIRECTION OF AN EXTERNALMAGNETIC FIELD COMPRISING: A PARAMAGNETIC SUBSTANCE HAVING ADETERMINABLE MAGNETIC RESONANCE ABSORBTION FREQUENCY AND MAGNITUDECHARACTERISTIC; FIRST GENERATOR MEANS FOR IMPOSING ON SAID PARAMAGNETICSUBSTANCE A HIGH FREQUENCY MAGNETIC FIELD, SECOND GENERATOR MEANS FORIMPOSING ON SAID SUBSTANCE A LOW FREQUENCY MAGNETIC FIELD PARALLEL TOSAID HIGH FREQUENCY FIELD AND HAVING A FREQUENCY AND A MAGNITUDE LESSRESPECTIVELY THAN THE CHARACTERISTIC MAGNETIC RESONANCE ABSORPTIONFREQUENCY AND MAGNITUDE OF SAID PARAMAGNETIC SUBSTANCE, AND THIRDGENERATOR MEANS FOR IMPOSING ON SAID SUBSTANCE A MAGNETIC FIELD OFCONSTANT MAGNITUDE LESS THAN THE MAGNETIC RESONANCE ABSORPTION MAGNITUDEOF SAID PARAMAGNETIC SUBSTANCE ROTATING WITH A CONSTANT ANGULAR VELOCITYLESS THAN THE ANGULAR VELOCITY CORRESPONDING TO THE MAGNETIC RESONANCEABSORPTION FREQUENCY OF SAID PARAMAGNETIC SUBSTANCE IN A PLANEPERPENDICULAR TO THE AXIS OF SAID LOW FREQUENCY FIELD; AND MEANS FORMEASURING ASYMMETRICAL VARIATIONS IN THE ABSORPTION OF SAID PARAMAGNETICSUBSTANCE.